The present invention relates to a power supply control device for use with an electronic control system, and more particularly to a power supply control device having a self-holding circuit, for use with an automotive electronic control system.
Various automobiles ranging from passenger cars to large-size trucks are now being controlled by electronic control systems including microcomputers. One representative example of such an electronic control system is disclosed in U.S. Pat. No. 4,601,369. According to the disclosed system, the depth to which an accelerator pedal is depressed and the automobile speed are detected at all times, and a gear transmission position is determined from the detected pedal depth and speed for changing transmission gears. In synchronism with changing transmission gears, a clutch is also controlled by the control system as if human intervention were involved.
In this conventional electronic control system for such automotive control, when a key switch is turned on, a voltage is applied by a battery to a constant-voltage circuit in the electronic control system, and a voltage is applied by the constant-voltage circuit to energize the electronic control system in readiness for operation. When the key switch is turned off, the voltage applied to the constant-voltage circuit is cut off to make the electronic control system inoperative. Therefore, if the key switch is turned off in error or a fuse connected between the battery and the key switch melts, then the following problems will arise:
(1) An electronically controlled gear transmission, an electronically controlled clutch, and other devices that are controlled by the electronic control system will no longer be under control.
(2) Where a power steering system is of the speed-responsive type, operation of the power steering system to control the load on the steered wheels will be interrupted, making the steering wheel feel unduly heavy or light.